The present invention relates to a lithium secondary battery wherein the electron conductivity of the positive electrode active material layer is improved, the internal resistance is reduced, and discharging in large output and in large current is possible and which can be suitably used particularly as an electric source for driving of the motor of an electric vehicle or the like.
In recent years, as the movement for environmental protection has become active, it has become a serious issue to control the exhaust gas (e.g. carbon dioxide and other harmful materials) emitted from internal combustion engines or to save energy. In this connection, it has become common, in the automobile industry, to investigate the market introduction, as early as possible, of electric vehicles (EVs) or hybrid electric vehicles (HEVs) in place of conventional automobiles using fossil fuels (e.g. gasoline).
As the battery used for driving of the motor of EV or HEV, a lithium secondary battery is promising for its high energy density. In order for an EV or HEV to exhibit sufficient performance in acceleration, slope-climbing property, continuous running property, etc., the lithium secondary battery used therein must have a large capacity and a large output. For example, in an HEV, the lithium secondary battery for motor driving must have a high output because the motor assists the output of vehicle during acceleration. Since the voltage of a single battery is determined by the materials constituting the battery and, in the case of lithium secondary battery, is at best about 4.2 V in terms of open-circuit voltage and about 3 to 4 V in terms of actual discharge voltage, the above-mentioned xe2x80x9chigh outputxe2x80x9d means that a large current flows. In HEVs, etc., a large current of 100 A or more flows often and, in some cases, a current as large as 500 A flows in a short period of time.
To operate the motor of an EV or HEV, it is necessary to connect a plurality of single batteries in series to secure a required voltage. As a result, a current of the same amount flows through the individual batteries. In this case, if each individual battery has a large internal resistance, it is impossible to generate a current of a required amount. Further, if each individual battery has a large internal resistance, a large amount of Joule""s heat is generated owing to the internal resistance, and an increase in battery temperature and resultant vaporization of electrolytic solution may take place, which may incur the malfunctioning of batteries. Hence, in order to give rise to discharging of large current and high output such as mentioned above, it is necessary to reduce the internal resistance of each single battery.
With respect to the internal resistance of a single battery, analysis has been made on the resistance to electron conduction or ion conduction, of each material constituting the battery and, as a result, it has been made clear that the resistance to electron conduction, of positive electrode active material occupies a large portion of the internal resistance of a single battery. Hence, it has been attempted to add, to the positive electrode active material, a carbon material (e.g. acetylene black) as a conductivity-improving agent, as one means of reducing the internal resistance of a single battery. It is expected that by increasing the amount of acetylene black added, the electron conductivity of positive electrode active material layer is made higher and the internal resistance of the single battery is reduced.
Addition of acetylene black, however, has problems. Addition of acetylene black contributes to the improvement in electron conductivity but does not contribute to the increase in battery capacity; therefore, the addition reduces the energy density of battery. Further, being bulky, acetylene black is difficult to disperse in production of a slurry of positive electrode active material, and the resulting slurry has low uniformity and low coatability onto substrate.
To alleviate the above low coatability, it is considered to increase the amount of the binder used. This approach, however, invites further reduction in energy density and, moreover, the insulating property of the binder (the binder used has an insulating property in many cases) may reduce the conductivity of the positive electrode active material layer, which has been increased by the addition of acetylene black. Hence, the amount of the acetylene black added is preferably determined so that the amount is kept at the necessary but minimum level, the maximum improvement in electron conductivity is attained, and the internal resistance of battery is reduced, while consideration is taken into the particle shape of positive electrode active material powder.
The present invention has been made in view of the above-mentioned problems of the prior art.
According to the present invention there is provided a lithium secondary battery, comprising:
An internal electrode body including a positive electrode, a negative electrode, and a separator, the positive electrode and the negative electrode being wound or laminated via the separator, and
an organic electrolyte;
wherein the active material used in the positive electrode satisfies the following relation between the average particle diameter R (xcexcm) and the specific surface area S (m2/g):
6xe2x89xa6Rxc3x97Sxe2x89xa650
and an amount of the acetylene black added to the positive electrode active material satisfies the following relation with the specific surface area of the positive electrode active material:
Sxe2x89xa6Wxe2x89xa6S+5(Wxe2x89xa610)
wherein W is the amount of the acetylene black added to the positive electrode active material, expressed in % by weight based on the amount of the active material, and S has the same definition as given above and is expressed in m2/g.
In the lithium secondary battery of the present invention, the positive electrode active material satisfies preferably the following relation between the average particle diameter R and the specific surface area S:
6xe2x89xa6Rxc3x97Sxe2x89xa625
In the lithium secondary battery of the present invention, the positive electrode active material has an average particle diameter of preferably 1 to 50 xcexcm, more preferably 5 to 30 xcexcm; and has a specific surface area of preferably 0.1 to 5 m2/g, more preferably 0.2 to 2 m2/g.
The positive electrode active material is preferably a compound oxide composed mainly of Li and Mn; and the molar ratio Li/Mn of Li and Mn in the positive electrode active material is preferably larger than 0.5. In the positive electrode of the present lithium secondary battery, the active material and the conductivity-improving agent are appropriately combined so as to give a single battery having a battery capacity of 2 Ah or larger. The lithium secondary battery of the present invention can be used suitably particularly in an electric vehicle or a hybrid electric vehicle.